The term “Chronic Obstructive Pulmonary Disease” (COPD) is generally used to describe the disorders of emphysema and chronic bronchitis. Previously, COPD was also known as Chronic Obstructive Lung Disease (COLD), Chronic Airflow Obstruction (CAO), or Chronic Airflow Limitation (CAL). Some also consider certain types of asthma to fall under the definition of COPD. Emphysema is characterized by an enlargement of air spaces inside the lung. Hence, emphysema is an anatomic definition and it can only be presumed in a living patient. Chronic bronchitis is characterized by excessive mucus production in the bronchial tree. Chronic bronchitis is a clinical definition and denotes those individuals who meet criteria defining the disease. It is not uncommon for an individual to suffer from both disorders.
In 1995, the American Lung Association (ALA) estimated that between 15–16 million Americans suffered from COPD. The ALA estimated that COPD was the fourth-ranking cause of death in the U.S. The ALA estimates that the rates of emphysema is 7.6 per thousand population, and the rate for chronic bronchitis is 55.7 per thousand population.
Those inflicted with COPD face disabilities due to the limited pulmonary functions. Usually, individuals afflicted by COPD also face loss in muscle strength and an inability to perform common daily activities. Often, those patients desiring treatment for COPD seek a physician at a point where the disease is advanced. Since the damage to the lungs is irreversible, there is little hope of recovery. Most times, the physician cannot reverse the effects of the disease but can only offer treatment and advice to halt the progression of the disease.
To understand the detrimental effects of COPD, the workings of the lungs requires a cursory discussion. The primary function of the lungs is to permit the exchange of two gasses by removing carbon dioxide from venous blood and replacing it with oxygen. Thus, to facilitate this exchange, the lungs provide a blood gas interface. The oxygen and carbon dioxide move between the gas (air) and blood by diffusion. This diffusion is possible since the blood is delivered to one side of the blood-gas interface via small blood vessels (capillaries). The capillaries are wrapped around numerous air sacs called alveoli which function as the blood-gas interface. A typical human lung contains about 300 million alveoli.
The air is brought to the other side of this blood-gas interface by a natural respiratory airway, hereafter referred to as a natural airway or airway, consisting of branching tubes which become narrower, shorter, and more numerous as they penetrate deeper into the lung. Specifically, the airway begins with the trachea which branches into the left and right bronchi which divide into lobar, then segmental bronchi. Ultimately, the branching continues down to the terminal bronchioles which lead to the alveoli. Plates of cartilage may be found as part of the walls throughout most of the airway from the trachea to the bronchi. The cartilage plates become less prevalent as the airways branch. Eventually, in the last generations of the bronchi, the cartilage plates are found only at the branching points. The bronchi and bronchioles may be distinguished as the bronchi lie proximal to the last plate of cartilage found along the airway, while the bronchiole lies distal to the last plate of cartilage. The bronchioles are the smallest airways that do not contain alveoli. The function of the bronchi and bronchioles is to provide conducting airways that lead inspired air to the gas-blood interface. However, these conducting airways do not take part in gas exchange because they do not contain alveoli. Rather, the gas exchange takes place in the alveoli which are found in the distal most end of the airways.
The mechanics of breathing include the lungs, the rib cage, the diaphragm and abdominal wall. During inspiration, inspiratory muscles contract increasing the volume of the chest cavity. As a result of the expansion of the chest cavity, the pleural pressure, the pressure within the chest cavity, becomes sub-atmospheric with respect to the pressure at the airway openings. Consequently, air flows into the lungs causing the lungs to expand. During unforced expiration, the expiratory muscles relax and the lungs begin to recoil and reduce in size. The lungs recoil because they contain elastic fibers that allow for expansion, as the lungs inflate, and relaxation, as the lungs deflate, with each breath. This characteristic is called elastic recoil. The recoil of the lungs causes alveolar pressure to exceed the pressure at airway openings causing air to flow out of the lungs and deflate the lungs. If the lungs'ability to recoil is damaged, the lungs cannot contract and reduce in size from their inflated state. As a result, the lungs cannot evacuate all of the inspired air.
In addition to elastic recoil, the lung's elastic fibers also assist in keeping small airways open during the exhalation cycle. This effect is also known as “tethering” of the airways. Such tethering is desirable since small airways do not contain cartilage that would otherwise provide structural rigidity for these airways. Without tethering, and in the absence of structural rigidity, the small airways collapse during exhalation and prevent air from exiting thereby trapping air in within the lung.
Emphysema is characterized by irreversible biochemical destruction of the alveolar walls that contain the elastic fibers, called elastin, described above. The destruction of the alveolar walls results in a dual problem of reduction of elastic recoil and the loss of tethering of the airways. Unfortunately for the individual suffering from emphysema, these two problems combine to result in extreme hyperinflation (air trapping) of the lung and an inability of the person to exhale. In this situation, the individual will be debilitated since the lungs are unable to perform gas exchange at a satisfactory rate.
One further aspect of alveolar wall destruction is that the airflow between neighboring air sacs, known as collateral ventilation or collateral air flow, is markedly increased as when compared to a healthy lung. While alveolar wall destruction decreases resistance to collateral ventilation, the resulting increased collateral ventilation does not benefit the individual since air is still unable to flow into and out of the lungs. Hence, because this trapped air is rich in CO2, it is of little or no benefit to the individual.
Chronic bronchitis is characterized by excessive mucus production in the bronchial tree. Usually there is a general increase in bulk (hypertrophy) of the large bronchi and chronic inflammatory changes in the small airways. Excessive amounts of mucus are found in the airways and semisolid plugs of this mucus may occlude some small bronchi. Also, the small airways are usually narrowed and show inflammatory changes.
Currently, although there is no cure for COPD, treatment includes bronchodilator drugs, and lung reduction surgery. The bronchodilator drugs relax and widen the air passages thereby reducing the residual volume and increasing gas flow permitting more oxygen to enter the lungs. Yet, bronchodilator drugs are only effective for a short period of time and require repeated application. Moreover, the bronchodilator drugs are only effective in a certain percentage of the population of those diagnosed with COPD. In some cases, patients suffering from COPD are given supplemental oxygen to assist in breathing. Unfortunately, aside from the impracticalities of needing to maintain and transport a source of oxygen for everyday activities, the oxygen is only partially functional and does not eliminate the effects of the COPD. Moreover, patients requiring a supplemental source of oxygen are usually never able to return to functioning without the oxygen.
Lung volume reduction surgery is a procedure which removes portions of the lung that are over-inflated. The improvement to the patient occurs as a portion of the lung that remains has relatively better elastic recoil which allows for reduced airway obstruction. The reduced lung volume also improves the efficiency of the respiratory muscles. However, lung reduction surgery is an extremely traumatic procedure which involves opening the chest and thoracic cavity to remove a portion of the lung. As such, the procedure involves an extended recovery period. Hence, the long term benefits of this surgery are still being evaluated. In any case, it is thought that lung reduction surgery is sought in those cases of emphysema where only a portion of the lung is emphysematous as opposed to the case where the entire lung is emphysematous. In cases where the lung is only partially emphysematous, removal of a portion of emphysematous lung increases the cavity area in which the non-diseased parenchyma may expand and contract. If the entire lung were emphysematous, the parenchyma is less elastic and cannot expand to take advantage of an increased area within the lung cavity.
Both bronchodilator drugs and lung reduction surgery fail to capitalize on the increased collateral ventilation taking place in the diseased lung. There remains a need for a medical procedure that can alleviate some of the problems caused by COPD. There is also a need for a medical procedure that alleviates some of the problems caused by COPD irrespective of whether a portion of the lung, or the entire lung is emphysematous.
The present invention addresses the problems caused by COPD by providing a device configured to create collateral openings through an airway wall which allows expired air to pass directly out of the lung tissue responsible for gas exchange. These collateral openings ultimately decompress hyper-inflated lungs and/or facilitate an exchange of oxygen into the blood.
Furthermore, there is also a need for devices that are able to access remote areas of the body to provide dual functions of locating an acceptable site for removal or cutting of tissue and then removing or cutting the tissue without having to reposition the device. Such a need is evident in dynamically moving environments (e.g., the lungs) where repositioning of a device to find the original target site may be difficult.
Doppler ultrasound is an effective means to determine the presence or absence of a blood vessel within tissue. It is known that sound waves at ultrasonic frequencies travel through tissue and reflect off of objects/interfaces where density gradients exist. In which case, the reflected signal and the transmitted signal will have the same frequency. Alternatively, in the case where the signal is reflected from the blood cells moving through a blood vessel, the reflected signal will have a shift in frequency from the transmitted signal. This shift is known as a Doppler shift. However, since the characteristics of components used to detect Doppler shift vary from characteristics of components used to cut or remove tissue, it is difficult to cut or remove tissue in precisely the same location and immediately after detection has taken place. It is usually required that the component or device used to detect any Doppler shift first must be moved to allow a second component or device to cut or remove the tissue at the same precise location.
For instance, if a device uses energy to create an opening or ablate tissue, the energy delivery components may not have acceptable characteristics to also serve as Doppler components. Furthermore, the process of delivering energy through the device may undesirably impact any Doppler components.
When using Doppler in tissue it is noted that the acoustic impedance of the ultrasound transducer and the acoustic impedance of tissue differ significantly. As a result, the ultrasound signal may experience significant reflection and divergence at the tissue/transducer interface. To address this issue, a tip or lens may be used as an interface between the transducer and tissue.
In common Doppler ultrasound applications, a tip material is selected to provide an optimum acoustic match between the ultrasonic transducer and tissue. This optimum acoustic match is the geometric mean impedance between the tissue and the transducer material, governed by the following equation.Zoptimum=(Ztissue×Ztransducer)½
Where Zoptimum is the desired acoustic impedance of the tip material; Ztissue is the acoustic impedance of tissue; and Ztransducer is the acoustic impedance of the transducer. Generally, Ztissue ranges from 1.38 MRayls (for fat) to 1.70 MRayls (for muscle), while Ztransducer is approximately 30 MRayls for ceramic transducer materials. Therefore, using Ztransducer of 1.54 MRayls (the average acoustic impedance for tissue) the desirable tip material should have an acoustic impedance around 6.79 MRayls.
Most materials having an acoustic impedance close to this range are made of epoxy composites and range from, for example, 1.78 MRayls for a methylpentene copolymer (e.g., TPX, Matsui Plastics, White Plains, N.Y.) to 4.39 MRayls for high temperature plastics (e.g., CELAZOLE, Curbell Plastics, Glenshaw, Pa.).
One drawback to using Doppler ultrasound devices for placing collateral openings in tissue is that conventional tip materials selected for their desirable acoustic impedance are not effective to deliver energy (e.g., RF, resistive heat, etc.) The acoustic impedance of electrically and thermally conductive materials is higher than the desired acoustic impedance of 6.79 MRayls. For example, Zaluminum is approximately 18 MRayls, Ztitanium is approximately 27 MRayls, and Zstainless steel is approximately 45 MRayls.
Another drawback to delivering energy through devices configured for Doppler applications is that the transducer is prone to being damaged. For example, when used to deliver therapeutic RF energy, an electrically conductive tip experiences heating. If a sufficient amount of heat is conducted from the tip, the transducer may depolarize. Moreover, conduction of heat through the device may adversely affect the joints and bonds between the transducer, tip and device. As a result, there is the potential of a catastrophic failure of the device if the assembly breaks apart during use in the body.
In view of the above, the present invention provides a device capable of locating an acceptable site for the creation of a collateral opening and then creating an opening in the tissue using a device capable of both functions. While the present invention is discussed as having applicability to creation of collateral openings it was found to have utility for other applications as well. For example, the present invention is suited for the application of energy to tissue in a safe manner (e.g., tumor ablation, tissue removal, application of heat to structures within the body, etc.). Especially when there is a need to avoid blood vessels, or other tissue/organs/structures. The invention has applicability given a need to use of Doppler effect to locate movement within tissue and then apply energy based on the observation of the Doppler effect.
Methods and devices for creating, and maintaining collateral channels are discussed in U.S. patent application Ser. No. 09/633,651, filed on Aug. 7, 2000; U.S. patent application Ser. Nos. 09/947,144, 09/946,706, and 09/947,126 all filed on Sep. 4, 2001; U.S. Provisional Application No. 60/317,338 filed on Sep. 4, 2001, and 60/334,642 filed on Nov. 29, 2001, whereas the entirety of each listed application is incorporated by reference herein.